Works here in grid mode. OS X 10.11.6 Lr 2015.8.
What is the language setting in Lr and keyboard?
Do Help > System Info to verify you are actually running CC 2015.8 / 6.8 -- LR often fools people into thinking they got the latest update.
If you are running 2015.8, then it may be that Adobe screwed up the fix to a bug with the "x" shortcut. Please add details about your issue to this bug report in the official Adobe feedback forum: Lightroom: x keyboard shortcut is assigned to two functions in Library module [Swedish language] | Photoshop Family Cust…
Works here with German language set.
I have the X shortcut issue, running Lightroom 6.8 in French under OS X.
CAPS+X works, but not X alone...
This has been acknowledged as an issue. Please see here for more info: Lightroom CC 2015.8: Shortcut X to reject an image in Library not available (French & Dutch) | Photoshop Family Customer…
Please update Lightroom to 6.9/2015.9 version which includes the fix for Lightroom CC 2015.8: Shortcut X to reject an image in Library not available (French) | Photoshop Family Customer Communi…